Acetylation of cellulose



May 2, l944 GIBSON ETAL 2,348,001

.AACETYLATION OF CELLULOSE Filed OCT.. 24, 1941 Patented May 2, 1944 Q1"2,348,001, l AcETYnATIoN oF-cnLLULosE. c f i Donald L. Gibson, westchester, and william M. Shoemaker, Kennett Square, Pa., assignors toYNational vulcanized Fibre a corporation of Delaware ton, Del.,

Company, Wilming- Application October 24, 1941, Serial No. 416,376 Y. r

1-2 claims. (o1. 8 121) This invention relates to the acetylation ofcellulose and particularly to the continuous acetylation of a travelingweb of cellulose material, such as paper or fabricated cellulose lamentsto effect a modication of the characteristics of the material uniformlyand progressively and thereby to produce a new material capable of usesfor which the original is not adapted.

While the invention is particularly useful in the preparation ofacetylated materials from paper stock, it has been found that desirableproducts may be produced also from fabrics produced from cellulosematerials by weaving or otherwise when such fabrics are subjected to themethod as hereinafter described. It has been suggested that a web ofpaper may be subjected to acetylation by leading it over suitable rollsthrough an elongated bath containing an acetylating agent and anonsolvent diluent. Nevertheless, acetylated paper in the form of websor sheets has not heretofore been available as a commercial product,and" industry has not had an opportunity to utilize such material forthe purposes for which it is particularly useful. In theory, suchmaterial may` be prepared without difficulty, but in practice there aremany problems which heretofore have baled those who sought to prepareacetylated paper as a commercial product. Consequently it has not beenpossible to manufacture acetylated paper on a commercial and economicalbasis.

Foremost amongthe problems is that .of retaining the fibrous form of thepaper or fabric in the acetylated product. Acetylation of cellulose is,if the acetylating agent is effective, a reaction which proceedsrapidly, and it is -very difficult to control this reaction which, ifuncontrolled, results in complete disintegration lof the cellulose.acetylation results immediately and during the treatment withv theacetylating agent in substantial loss ofl tensile strength. In theabsence of suitable precautions the material loses its fibrous structureand cannot be maintained in the formV of a sheet or web. A previouslysuggested method lof acetylating a web of paper produces completedegeneration and not a' web of acetylated paper as intended. i

Assuming a suitable bath which is properly adjusted to effectacetylation without disintegra tion, it is necessary, since the tensilestrength of the material is materially reduced, to avoid any stresswhich would tear the web, and thus prevent continuous travel thereof.Moreover, the acetylation of the material rapidly exhausts the aceticanhydride which is the acetylating agent, and as the material advancesthe acetylation formula becomes progressively less' effective until itfails. Underthis condition, the product is non-uniform and useless.

'Ihere are numerous other factors Whichmust be considered. These will bereferred to in more detail hereafter.v The continuous maintenance of theessential conditions for the acetylation of/Webs of cellulose materialhas not been possible heretofore since some of the conditions Awere Vnotrecognized and no attempt had been made to employ suitable control inrespect to other details.

It is the object of the present inventionftopro- .vide a novel method ofyacetylating a web of cellulose material in a continuous operation whicheffects a uniform treatment of the material in a minimum of time, thusaffording a practical commercial operation permitting the economicalpreparation of new products having advantageous properties andcharacteristics which make them useful for various purposes.

Another object of theinvention is the provision of a method wherein aweb' of cellulose material is advanced continuously and subjected touniform acetylation without the introduction of stresses which wouldbreak the web in its weakened condition. Another object of the inventionis the provision 4of 'a methodwherein the web of cellulose mate- In thecase of fibrous material,

-rial .is subjected to an acetylating formula capable of rapidly andeffectively modifying the material as it advances, the. operation beingcontrolled to avoid variations in the amount of acetylation effected insuccessive portions of the web. l

Another object of the invention is the provision of a method in whichthe uniformly acetylated web is purified continuously Vand effe'ctivelyto aiordnew and satisfactory products which will not deteriorate in use.

-val, purifying procedure and When all of these factors areproperlycorrelated of uniform concentration. The

, itate purification.

' maintain it at its maximum concentration so Another object of theinvention is ther provision of a method of effectively controlling theconcentration ofthe acetylating agent so -that the invention is theproby reference to the following' specification and accompanyingdrawing, in which Fig. 1 is a diagrammatic illustration in longitudinalsection of an apparatus adapted for the practice of the invention; and

Fig. 2 is-a section on the line 2-2 of Fig. 1. The apparatus hereinafterdescribed is not in all of its details essential to the invention. vItis.

therefore, merely illustrative of a preferred embodiment of practicalmeans for accomplishing the desired results, which has been usedsuccessfully in a plant operation. `Various modifications of such anapparatus may be used to accomplish the purpose of the invention,provided the essential conditions of operation are observed.

In carrying out the invention, the following variables govern thetreatment and the characteristics`of the nished product: type ofmaterial treated, conditionsof pretreatment with acetic acid includingtemperature and time of immersion of the paper therein, strength ofacetic anhydride in the acetylating bath, percentage of catalyst in thebath, maintenance of uniform composition of the bath, composition ofaddition agents employed to maintain uniformity in the bath, time ofimmersion and temperature of the acetylating bath, ratio of the totalacetylating bath to the amount of paper being treated at any interdryingprocedure.

in accordance with their relative importance, it is possible to securerapid and effective modification of the characteristicsof the cellulosematerial Without deterioration thereof and to maintain continuousoperation. n

The procedure is conducted preferably in an apparatus in which subjectedto a prelixnlnary treatment or soaking in acetic acid of suitableconcentrationl and there'- after-subjected to the action of aceticanhydride of suitable concentration and .at a proper temperature in thepresence of a catalyst and under conditions in which both sides of theweb are subjected progressively to an acetylation formula web advancescontinuously, without being subjected to ariy substantial stress which.would tend totear it in its weakened condition, until it is deliveredto a bath of water which promptly quenches the reaction andrestorespartially the tensile strength of the paper. This water bath should beconstantly renewed and may contain suitable agents to facil- The web isthen led through a succession of water'baths which aresimilarlyfreshened and thence to the dryers.

To maintain uniformity of acetylation it is necessary to circulateconstantly the acetylation formula, with suitable additions to fortifyand the cellulose material is first each increment of the web as itcomes from the soaking bath is uniformly aected by the acetylatingformula at its maximum strength, that is to say, each increment of theweb as it advances receives precisely the same treatment.

In order that the operation may be morefcl'early understood, referenceis made to the accompanying drawing, in which 5 indicates a rollorcellulose material to be treated. which is mountedon a spindle 8.Unrolling of the web-is retardediby a suitablel brake indicateddiagrammatieally'y by' the weight 1. The web 8 is delivered toareceptable 9 which may be made of any suitable material which is notaffected by the soaking bath .I8 consisting of acetic acid. Provision ismade for the addition and likewise to'v maintain the bath at apredetermined temperature. The web 8 passes under rollers II and I2 andis drawn through the soaking bath by driven rollers I3 and I4 whichsqueeze the excess of acetic acid from the paper as itadvarices. The webis fed by the rollers I3' and I4 to the acetylation chamber comprisingan inclined plane surface I5 enclosed by a suitable hood I8 having anextension I1 covering the soaking bath Ill and an extension I8 whichdips into the iirst water bath I9 so that fumes from the acetic acid andacetic anhydride are prevented from escaping to the surroundingatmosphere.

The angle of the inclined plane I5 is such that the web will traveldownwardly thereover under the inuence of the acetylating agent whichflows continuously with the web. The web advances continuously withoutbeing subjected to any stress which is likely to tear it, a conditionwhich cannot be maintained if an attempt is made to draw the web througha bath by the application of tension. The angle of the inclined plane I5may be varied considerably depending upon the nature of the material,but an angle of approximately 22 from the horizontal has been found tobe satisfactory with most types of webs.

The acetylating agent flows with the web into a receptacle 20 from whichit is withdrawn continuously through a` pipe 2I and delivered to astorage tank 22, provided preferably with a heat-A ing bath 22'. Watermay be used for heating to a constant temperature. Suitable provision ismade for the addition of materials to the storage tank from a tank 23'to maintain a precise composition of the acetylating agent at a uniformtemperature. .From the tank 22 the acetylating agent is withdrawncontinuously through a pipe 28 by a pump 24 and delivered by a pipe 25to a trough 26 and by a branch 21' to a trough 28.

l The troughs 2i and 28 overilowcontinuously, supplying the acetylatingagent beneath and above the web 8 as it advances to and down theinclined plane I5. This arrangement ensures that each increment of theweb is subjected under precisely uniform conditions to an acetylatingagent of exact formula. Also, the amount of acetylating agent'overowingfrom the troughs 26 and 28, beneath and with which the liquid from thosetroughs flows down the inclined surface I5 is such as to maintain theweb suspended inthe acetylating agent,

I- as shown in Fig. 1, so that both sides of the web are subjected totreatment with the acetylating agent that as it passes over the inclinedsurface and acetylation of the web proceeds uniformly and cannot varysubstantially in respect to the predetermined conditions.

In the' receptacle 28 thus avoiding any possibility the web s forms 'aioop. ze,

of substantial tenof acetic acid as required above the web, and theforce causes the web to advance and at the same time' removes the excessof the acetylating agent. At this point there is, of course, a certainloss of the acetylating agent due to drag out, which loss must betakeninto account in maintaining the precise strength of the acetylatingformula by suitable additions thereto.- The web passes into the Water inthe receptacle I9 which promptly quenchesthe acetylating reaction.Passing under rollers 32 and 33, the web is drawn out of the bath I9over rollers 34 land 35 and thence into successive Water baths 36 and 31.under rollers 38 and 39, over rollers 40 and 4|, under rollers 42 and43, over roller 44- and thence to a series of drying cans 45 where theweb is subjected to drying under regulated conditions. As hereinbeforeindicated, the water in the receptaclesi9, 36 and 31 is constantlyrenewed in order to elect rapid and complete purification, it beingessential that all of the acetylating agent be removed from the papertogether with any other impurities l which may be present.

'The conditions other than those already mentioned, under which theoperation is conducted. involve particularly the character and strengthof the treating solutions as well as the temperature and time oftreatment.- For the soaking of the web preliminary to acetylation,weprefer to use commercial glacial acetic acid of a purity approximating99.5%. It is believed that the presence of a minute amount of waterequivalent to approximately 0.5 is necessary. Up to 2% of water in theacetic acid is permissible. The temperature of the pre-soaking bath ispreferably between 80 and 110 F., or from 5 to 10 F. lower than thetemperature maintained for the acetylating agent. The time of pre-soakwill vary with the degree of acetylation desired, which is controlled bythe speed of travel of the web. It will be usually one-third to one-halfthe time required for acetylation. As hereinbefore indicated, provisionis 'made for the maintenance of the temperature and concentration of thepresoaking lbath so that it will remain uniform during the entireoperation, thus eifecting uniformity in the treatment of the web as itadvances progressively.

The acetylation formula consists preferably of a liquid vhaving thefollowing composition by weight:

Percent Acetic anhydride (97% technical grade) 50.75 Acetic acid (99.5%glacial) 49 Catalyst-perchloric acid (dry basis) 0.25

agent at uniform concentration. This will necessarily vary dependingupon conditions, but a suitable strengthening formula consists of aceticanhydride 64.5%, acetic acid 35.25%, and perchloric acid 0.25%. Thepercentages mentioned are by weight, and the materials are thosedescribed in connection with the formulation of the acetylating agent.Obviously the acetylating agent must be subjected continuously tocontrol by suitable analyses, and changes will be made in thestrengthening formula depending upon the conditions observed. The timeof treatment will vary, depending upon the degree of acetylationdesired, from 2 minutes to approximately 12 minutes. Economy inoperation, that is to say speed of treatment and corresponds to thespeed of travel of the web, and it is therefore desirable to limit thetime of treatment to the minimum.

Purification is effected in three or more con- 'secutive tanks, allsupplied with circulating and refreshed water at approximately 100 F.The first purifying tank containing water stops thc reaction andcoagulates or, hardens the material,

thus restoring its tensile strength. The second,

tank will include approximately 0.1% of a suitable wetting agent such asTergitoL a sodium secondary-alcohol sulphate (The Journal of Industrialand Engineering Chemistry, volume XXIX, 1937, page 1234). The functionof the wetting agent is to facilitate the removal of impurities, and anyothersuitable wetting agent may be used.- The third tank contains purewater to complete purification of the material.

` The web is purified to a pH of approximately 6.5,

it being important tor remove all free acid and ionizable salts which,if retained, would interfere with the electrical properties of thematerial under humid conditions. Any number of purifying tanks, requiredto effect the desired purification of the web,` may be used.

The web is dried continuously by passing over v' copper drying canssurfaced with a phenolic resin. It is dried at about 220 F. untiloptimum electrical values are reached. It is important to dry thematerial properly, since the bres seem to set with proper drying,producing high elec'- trieal values. The speed of drying and the dryingtemperature are limited by the size of the equipment and the temperatureavailable. drying temperature is subject to wide variation andtemperatures stated herein are merely illustrative of the preferredprocedure. Higher temperatures could be used and the drying operationcorrespondingly limited with respect to time.

While we prefer to use perchloric acid as the catalyst, we may employany ofthe known acetylation catalysts such as sulphuric, hydrochloric,hydrobromic and trichloracetic acids, salts such as copper chloride,copper sulphate, copper perchlorate, zinc chloride, zinc acetate, zincsulphate and meta potassium periodate, and neutral esters such asdimethyl and diethyl sulphate. Certain of the catalysts mentioned, suchas the sulphuric acid, perchloric acid and meta potassium periodate, aremore active and. rapid than others. Sulphuric acid is so active andrapid that unless it is used in very small quantities and the conditionsof the reaction are very carefully controlled as to time andtemperature, the final product may not have good mechanical andelectrical properties.

suitable in the case of cellulosic materials such as cotton rag papersand high alpha pulp papers. When meta potassium periodate is used, itshould be mixed with an equivalent amount of a mineral acid to liberatemeta periodic acid, which appears to be the active catalyst.

The procedure as described is adaptable for use The Zinc chloride is al.fairly slow-actingcatalyst and is particularly l E. Purifying withvarious types of papers, but is particularly useful inthe treatment ofkraft papers and produces therefrom products of excellent quality. Themost strikingv characteristic of the product produced by the methoddescribed for the treatmentof various kinds of papers is the relativelyhigh volumetric resistivity of the paper under humid conditions whichmay vary from 5000 megohm inches up to or beyond 10,000,000 megohminches. This remarkable increase in volumetric resistivity adapts theproduct to numerous applications in the electrical field, as for examplein the production of condensers and other similar electrical devices.

To amplify the foregoing description, we have included the followingexamples. These examples aii'ord detailed description of procedureswhich we have followed successfully to produce satisfactory products ina continuous and readily maintainable operation. The examples, areillustrative and are not to be considered as limiting the disclosure ofthe invention.

Example I A. Paper A cotton paper made from uncooked raw rags.

Thickness-.0055" ,Basis weight-60# (24 x 36--500) Viscosity-66400centipoises. (The test for viscosity is the standard method developed byTappi.) pH-G Percentage of alkalinity in paperlow Bath used fortreatment Acetic anhydride (97% technical grade)- 50.7 Acetic acid(99.5% glacial) 49% Catalystperchloric acid-0.25% (dry (This ispercentage by weight) Strengthening formula added Acetic anhydride-64.5

` Acetic acid-35.25%

Perchloric acid-0.25%.

weight) Treatment conditions Presoak-l00% acetic acid at 80 to 85 F.Treating time-Zl/z minutes immersion in bath at 97 F.

basis) (Composition by 'I'he treated paper is dried continuously overcopper drying cans surfaced with phenolic resin.

Paper is dried at about 220 F. until optimum electrical values arereached. d

G. Descriptiorrof the acetylated paper produced by the above method I'hepaper was treated to a basis Weight (24 x 36-500) on a dry basis of66-67 pounds,

B. Bath used for treatmentv Same as in Example 1 C. Strengtheningformula added Approximately asin Example 1 D. Treatment conditionsPresoak-100% acetic acid at (l0-85F. Treating time-3% minutes immersionin bath at 93 to 94 F.

E. Purifying Same as in Example 1 F. Drying conditions Same as inExample 1 G. Description of the acetylated paper produced and had apercentage combined acetic; acid of i 19-20 per cent. When tested inhumid air at 90% R. H. and 25 C. the paper hada volumetric resistivityof6,000 to 7,500 megohm-inches.

A. Paper Same as in Examplel l by the above method The paper wastreatedto 70-71 pounds basis weight (24 X 36-500) on a dry basis and apercentage combined acetic acid of 231/2 to 241/2 per cent. Theacetylated paper when exposed to humid airat R. H. at 25 C. has avolumetric resistivity of 60,000 megohm-inches.

Example 3 A. Paper A. saturating paper made from kraft (A11 compositionby Paper was acetylated to 75 pounds basis weight (25 x 36-500) and apercentage combined acetic acid of 45 to 46 per cent. v'Ihe acetylatedpaper when exposed to humid air at 90% R. H. at 25 C. had a. volumetricresistivity of 1,250,000 and 1,500,000 megohm-inches.

(All composition by acetylated paper when exposed'to humid air atv 90%R. H. at 25 C. had a volumetric resistivity of 20,000 megohm-inches.

Example Paper A light weight paper made fibres (S424) Thickness-.003Basis weight-30 to 31 pounds (24 x 36-500) Viscosity-44 centipoises pH-6Percentage alkalinity-low B. Bath used for treatment Same as in Example1 C. Strengthening formula. added Acetic anhydride-69.5%

Acetic acid-30.25%

Perchloric acid-0.25%. (All composition by Weight) v Treatmentconditions Presoak--100% acetic acid at 85 to 88 F.

Treating time-10 minutes immersion in bath at 96 to 97 F.

Purifying Y Same as in Example 1 F. Drying conditions Same as in Example1 G. Description of the acetylated paper produced by the above methodThe paper Was acetylated to a basis weight of 46 to 47 pounds (24 x36-500) and a degree of combined acetic acid of 45 to 46 per cent. Theacetylated paper when exposed to vhumid air at 90% R. H. at 25 C. had avolumetric resistivity of 500,000 megohm-inches.

Example 6 f from 100% kraft The paper was acetylated to a basis weightof 38 to 39 pounds (24 x 36-500) and to a per cent combined acetic acidof approximately 135%. When exposed to humid air at 90% R. H. at 25 C.the acetylated paper had a volumetric resistivity of' 10,000 megohminches.

The treatment of cotton cloth in accordance with the invention producesequally desirable products. Following are examples of such treatment.

The runs were made making use of our standard metering device andstandard amount of bath (160 lbs.). After treatment,'the material (Allcomposition by was purified with water by using a system of'7`5 threetanks (with circulation) and 0.1% Tergitm solution. Purification wascontinued until a pH of six or better was reached. The acetylated clothwas dried in continuous rolls over can dryers, the drying beingcompleted at temperature of 230 F. to aiord optimum electrical prop--erties. After drying, the cloth was tested-for electrical properties. atrelative humidity, 25 C., for 24 hours, and for mechanical properties.

Example 7 A. Cloth Puried cotton fabric (0-40) Thickness .008 2".0084

Weight 3.16 oas/sq. yd.

Count 80x 80 Viscosity to 179 cps.

Section A-2 minutes at 100 F. Section B-3 minutes at 100 F. SectionC-31/2 minutes at 100 F. ,f Section D-41/ minutes at 110 F. Section E-8minutes at 110 F. Section F-lll/z minutes at 110 F. Section G-2 minutesat 100 F.

Cloth was puried and dried.

Eample 8 Cloth- Same as in Example 7 Treating bath Per cent by weight(97.5% tech. grade) acetic anhydride- 49.25 (99.5% pure) glacial aceticacid 50.50 (Solid bis) perehloric acid 0.25

i i l 100.00 Treating conditions Ran 6 sections, designated A to F.Presoak and temperatures of presoak 1. A, B, C-% acetic acid at 110 F.2. D, E, F-98% acetic acid and 2% Water' at F. Immersion time in bathand temperature of bath Section A--5l minutes at 110 F. Section B-8minutes at 110 F. Section C-111/2 minutes at 110 F. Section D-5 minutesat 110 F. Section E -8 minutes at 110 F. Section F111/2 minutes at 110F.

D. Cloth was purified anddried.

and directing it perior properties. The untreated cloth had volumetricresistivity in megohm-inches of 42, and after treatment a maximum of3,500,000. The insulation resistance of the untreated cloth in megohmswas 189, and after treatment reached a maximum of 3,000,000.

lThere was no substantial loss of' mullen or' tensile strength in any ofthe samples, and in most cases a gain in these factors. The acetylationin terms of per cent of combined acetic acid varied from 14.5 to 40,best results being indicated With acetylation in excess of 35%.

Various changes may be made in the apparatus and in the procedure as'described without departing from the invention or sacrificing theadvantages thereof.

' We claim:

1. The method of acetylating cellulose material of the web to anacetylating agent of predetermined uniform composition, and maintaininguniformity of the acetylation of the web by continuously withdrawing andfortifying a -portion of the acetylating agent and returning saidfortified portion to contact wtih successive increments of thenon-acetylated portion of the web.

3. The method of acetylating cellulose material which comprisescontinuously advancing a web of the material, subjecting each incrementof the web to initial contact with an acetylating agent of predetermineduniform composition owing in the direction of travel of the Web,

withdrawing and fortifying a portion of the acetylating agent, andreturning the fortified portion of the acetylating agent for contactwith l successive increments of the nonacetylated portion of the web.

4. The method of acetylating cellulose material which comprisescontinuously advancing a web of the material, subjecting it to treatmentwith acetic acid, thereafter applying an acetylating agent to eachincrement of the advancing web, floating the web with the owinglacetylating agent downwardly over an inclined surface thereby avoidinglongitudinal stress in the web while it is subjected to the acetylatingagent, maintaining the composition of the acetylating agent by addingfresh reagents thereto, ensuring .uniform treatment of the material bycirculating the refreshed acetylating agent and directing it intocontact with the non-acetylated incre' ment of the advancing web, andfinally washing the web.

5. The method of acetylating cellulose material which comprisescontinuously advancing a web of the material, subjecting it to treatmentwith acetic acid, thereafter causing it to pass downwardly over aninclined surface, supplying lrial which comprises with such force as tomaintain the web suspended in the acetylating agent as it passesdownwardly over the inclined surface, and finally washing the web withwater. l

6. The method of acetylating cellulose material which comprisescontinuously advancing a web of the material, subjecting it to treatmentwith acetic acid, thereafter causing it to pass downwardly over aninclined surface, supplying an acetylating agent containing at least 49%by weight of acetic anhydride both beneath and above the web as itpasses downwardly over the inclined surface, the acetylating agent beingsupplied beneath and above the web in such amounts and with such forceas to maintain the web suspended in the acetylatingv agent as it passesdownwardly over the inclined surface, and finally washing the web withwater.

7. The method of acetylating cellulose material which comprisescontinuously advancing a web of the material, subjecting it to treatmentwith .acetic acid, thereafter causing it to pass downwardly over aninclined surface, supplying an acetylating agent both beneath and abovethe web as it passes downwardly over theinclined'surface, theacetylating'agent being supplied beneath and above the web in suchamounts and with such force as to maintain the web suspended in theacetylating agent as it passes downwardly over the inclined surface, andfinally washing the web with water, and maintaining the composition ofthe acetylating agent by adding fresh reagents thereto.

8. The method of acetylating cellulose material which comprisescontinuously advancing a web of the materiaL'subjecting it to treatmentwith acetic acid, thereafter causing it vto pass downwardly over aninclined surface, maintaining the web in contact with an acetylatingagent as it passes downwardly over the inclined surface, and finallywashing the acetylated web with water, maintaining lating agent byadding fresh reagents thereto, and insuring uniform treatment of thematerial bycirculating the refreshed acetylating agent and directing itinto contact with the lnonacetylated increment of the advancing web.

9. The method of acetylating cellulose matecontinuously advancing a webof the material, subjecting it to treatment with acetic acid, thereaftercausing it to pass downwardly over an inclined surface, supplying anacetylating agent both beneath and above the web as it passes downwardlyover the inclined surface, the acetylating agent being supplied beneathand above the web in such amounts and with such force as to maintain theweb suspended in the acetylating agent as it passes downwardly over theinclined surface, and nally washing the` web with water, maintainingthe'composition of the acetylating agent by adding fresh reagentsthereto and insuring uniform treatment of the material by circulatingthe refreshed acetylating agent and directing it into contact with thenonacetylated increment of the advancing web.

10. The method of acetylating cellulose material which comprisescontinuously advancing a an acetylating agent both beneath and above yweb of the materialysubjecting it to treatmentv with acetic acid,thereafter causing it to pass downwardly over an inclinedsurface,vsupplying an acetylating agent both beneath and above the webas it passes downwardly over the inclined surface,v the acetylatingagent being supplied beneath and above the Yweb in such amounts and withsuch ,force as to maintain the web suspended the composition of theacetywith acetic acid, thereafter causing it to pass downwardly over`ari inclined surface, supplying an acetylating agent both beneath andabove the web as it passes downwardly over the inclined surface, theaoetylating agent being supplied beneath and above the web in suchamounts and with such torce as to maintain the web sus pended in theacetylating agent as it passes downwardly over-,the inclined surface.the ineline of the web during its passage over the inclined surfacebeing such that it advances without longitudinal stress, and iinallywashing the web with water, maintaining the composition of theacetylating agent by adding fresh reagents thereto and insuring uniformtreatment of the material by circulating the refreshed acetylatinlgagent and directing it into contact with the nonacetylated increment o!the advancing web.

12. Ihe method of acetylating cellulose material which comprisesadvancing a web o! the material, subjecting it to treatment with aceticacid, thereafter causing the web to pass through an acetylating zone ina direction such that the force ot gravity normally would cause a83881118 of the web, supplying an acetylating agent both beneath andabove the web as it passes through said acetylating zone, theacetylatingagent being supplied beneath and above the web in suchamounts and with such force as to maintain the web suspended in theacetylating agent as' it passes through the acetylating zone.

